1. Field of the Invention
The present invention relates to a circuit for the controlled return of a pointer driven by a stepping motor in the event of failure of the supply voltage, where the circuit arrangement has a control circuit for actuating the stepping motor. The preferred application of the invention relates to an analog display apparatus in a combination instrument in a vehicle, where the display apparatus indicates a measured variable significant to the operational reliability of the vehicle such as, for example, the brake pressure indicator in the combination instrument in a commercial vehicle.
2. Description of the Related Art
An analog display apparatus for a vehicle with a pointer driven by a stepping motor is known, for example, from UK patent application no. GB 2 140 922, in which an electromagnetically driven rotor in the stepping motor drives the axle of the pointer. In this type of display apparatus, the pointer is almost always driven using a step-down drive mechanism. The step-down drive mechanisms used may, for example, include spur-gear drive mechanisms, which are sometimes even of multistage design, worm drive mechanisms, or spindle drive mechanisms. The drive mechanism and the natural restraint of the stepping motor""s rotor, which is of permanent-magnet design, prevents a return spring to return the pointer to its zero position or to another indicated value which is noncritical in relation to the scaling on the display apparatus, as can be done in display apparatuses which are driven by a crossed-coil measuring element or a moving-coil measuring element, which themselves have a return spring by virtue of their principle.
Analog display apparatuses having a pointer driven by a stepping motor therefore have no defined home position in the deenergized or unactuated state. This is because the pointer driven by a stepping motor in these display apparatuses is positioned using digitally calculated angular increments, so that the pointer also needs to be returned to its zero position in relation to the scale on the display apparatus by a corresponding control circuit for the stepping motor. This positioning of the pointer is assisted, for example, by a mechanical stop for the pointer which projects normal to the scale on the display apparatus.
If an unplanned or suddenly occurring failure of the supply voltage to the stepping motor""s control circuit occurs (such as a failure triggered by overloading of the on-board power supply), the control circuit, which usually comprises a microcontroller, does not have the time needed to return the pointer driven by the stepping motor to its zero position or to another indicated value which is noncritical in relation to the scaling on the display apparatus. That is, the pointer stays in the last position it adopted above the scale on the display apparatus before the failure of the supply voltage. If this display apparatus is used to indicate a measured variable significant to the operational reliability of the vehicle such as, for example, a display apparatus indicating the brake pressure in the combination instrument in a commercial vehicle, the pointer needs to indicate a noncritical value upon failure of the supply voltage. In the cited example, a brake pressure of zero bar thus needs to be indicated.
The object of the present invention is to provide a circuit for the controlled return of a pointer driven by a stepping motor in the event of failure of the supply voltage.
The object of the present invention is achieved by a circuit for the controlled return of a pointer driven by a stepping motor in response to a failure of the supply voltage. The circuit includes a display apparatus having a scale arranged thereon and a pointer movably connected to the display apparatus for moving relative to the scale for indicating a value on the scale. A stepping motor is operatively connected to the pointer for moving the pointer relative to the scale. A control circuit actuates the stepping motor, the control circuit receiving a supply voltage.
The circuit according to the present invention includes at least one buffer capacitor arranged for buffering at least the supply voltage for the control circuit for the stepping motor. The capacitance of the at least one buffer capacitor is sized to supply the control circuit for the stepping motor with electrical power at least until the control circuit has moved the pointer driven by the stepping motor to a zero position in relation to the scale on the display apparatus or to another indicated value which is noncritical in relation to the scaling on the display apparatus. The dependent claims then relate to further refinements and developments of the solution found.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.